There are a lot of big old buildings in Baltimore, with complex plumbing systems — just the kind where a common and potentially dangerous bacterium lurks.

It’s one that causes Legionnaires’ disease and can be hard to pinpoint and harder to eradicate, so it may not be surprising that State Center, the aging complex of buildings north of downtown, hasn’t been able to flush it out.

What is the bacterium, and how common is it?

Legionella exists naturally in water bodies and moist soil and can find its way to water systems, hiding in faucets, showerheads, water heaters and plumbing.

It becomes a problem when the bacteria grow unabated, particularly in stagnant water or where disinfectant hasn’t reached.

The most common way people are sickened is by breathing in tiny droplets from water sources such as water fountains, according to the U.S. Centers for Disease Control and Prevention. The bacteria are not spread person to person.

Can it make you sick, and who is most at risk?

Most people are not sickened by Legionella, the CDC says. When it does make people ill, usually two days to two weeks after exposure, they typically develop flu-like symptoms.

Legionnaires’ disease, a kind of pneumonia, can be dangerous for older people, smokers and people with other health issues that weaken their immune systems. That includes cancer, lung conditions and kidney or liver failure.

The threat to certain people has drawn attention from the U.S. Occupational Safety and Health Administration, which keeps an eye on workplaces with those big plumbing systems. It reports up to 18,000 people a year are hospitalized nationally from Legionnaires’ disease, though it can be tough to distinguish it from other types of pneumonia.

It’s diagnosed through a urine or blood test and treated with antibiotics.

In Maryland, there are typically 100-300 cases a year, according to the Maryland Department of Health. There were 170 cases last year, the latest data shows.

How is it prevented?

OSHA says proper maintenance is key to preventing the bacteria from growing and becoming a threat.

Specifically, officials there say keeping workers from being exposed depends on a water management program that identifies areas where Legionella could grow, decides on where control measures are needed and monitors them.

They also say building managers need to regularly monitor for elevated levels of the bacteria and have a plan when control measures fail.

Those concerned about the bacteria should avoid places where elevated levels of Legionella has been found or is suspected. That can be tough given some of these places, such as State Center, are workplaces. But workers, OSHA says, can drink bottled water and steer clear of the fountains or gym showers.

How is Legionella detected, and what is done when levels are elevated?

The Maryland Department of Health typically investigates, often with local or federal partners, when the bacteria are found through routine testing or after someone has become sick and tested positive. There haven’t been official reports of people sickened at State Center, though state officials did not respond to requests for comment.

In this case, the state’s Department of General Services is involved because the agency is in charge of the publicly owned buildings.

The department would be expected to flush out and clean the plumbing, and continue to monitor the water.

The American Federation of State, County and Municipal Employees — a union with an estimated 500 employees in the complex — released a statement calling on officials to give workers more information and provide them with more options to work outside of the complex.

“Sadly, these building issues are not new and they’re going to continue to happen in buildings across the state given the massive maintenance backlog we’re facing,” the statement said.

“We firmly believe all AFSCME members in the complex who can telework should be allowed to do so and provided the equipment necessary,” it said. “Others should be relocated swiftly, and those who must stay in order to temporarily shut the building down should be given the proper PPE, bottled water and emergency pay needed to safely be in the buildings.”